... some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness ... , durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium ... melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds ...